Polyamide carpet yarns usually are manufactured from continuous filaments or staple fibers that are already crimped of that have the ability to self-crimp upon being subjected to a heat treatment in a relaxed condition. The most commonly used polyamides for this purpose are polyhexamethylene adipamide (i.e., nylon 66) and polycaproamide (i.e., nylon 6). As a result of the crimp, the carpet yarns are bulky and provide carpets with the desired covering power, resilience and softness.
Numerous processes are known in the art wherein a melt-spun, quenched and drawn polyamide fiber is subjected subsequently to a special mechanical operation to produce a bulky yarn. The book by B. Piller, "Bulked Yarns", The Textile Trade Press, Manchester, England (1973) provides an excellent review of many of these known mechanical operations, including twist-texturing, stuffer-box crimping, knit de-knit texturing, gear-wheel crimping, pin texturing and edge crimping. Although each of these methods provides useful crimp to the fibers, each requires additional equipment, capital and energy beyond that required for the usual melt-spinning, quenching and drawing steps of synthetic fiber production. Furthermore, many of these mechanical operations often damage or weaken the fibers.
Piller also discloses the production of non-stretch bulked yarns by air-texturing methods. In these methods, a yarn is treated with a jet of compressed air which separates the individual filaments of the yarn and forms them into a looped structure. While being entangled into loops the yarn becomes shorter and bulkier.
A particularly useful technique for commercially preparing carpet yarns is jet-screen bulking of the type disclosed by Breen et al., U.S. Pat. No. 3,854,177. This technique provides a random, three-dimensional, non-helical curvilinear crimp to the fibers by passing a continuous filament yarn through a hot-fluid jet and onto a foraminous surface.
Processes have also been suggested for preparing crimped polyamide fibers which do not require special additional mechanical treatments subsequent to the fiber production steps. These processes involve specific melt-spinning or quenching techniques and provide the fibers with the ability to self-crimp when subjected to a heat-treatment in a relaxed condition. Included among these techniques are high speed spinning, jet quenching, special asymmetric liquid cooling, spinning of bicomponent fibers and spinning of fibers of asymmetric cross-section.
Bowling, U.S. Pat. No. 2,957,747, discloses a high speed spinning technique for providing spontaneously crimpable polyamide fibers which on relaxed heat treatment form small, irregular undulations. Bowling discloses melt-spinning, cross-flow quenching and attenuating velocities (without any mechanical drawing) of 3,000 to 6,000 yards per minute. However, Bowling notes that polyhexamethylene adipamide yarns made in this manner must be relaxed within a few minutes after attenuation (i.e., before significant tension is applied to the filaments) if the yarn thereafter is to be spontaneously crimpable.
Kilian, U.S. Pat. No. 3,118,012 suggests that a high velocity air jet directed against melt-spun polyamide filaments close to the face of the spinneret can provide filaments which are spontaneously crimpable. However, such high air velocities can cause problems with threadline control and denier uniformity.
A special liquid-cooling method for producing crimped fibers is suggested by Boyes et al., U.S. Pat. No. 4,038,357. In the suggested method, hot melt-spun filaments are initially partially cooled evenly by a radial outflow of air, starting at the spinneret and extending for 3 to 10 inches below the spinneret, and then further cooled by contact with a liquid film, which is thinner than the filaments, in such a manner that one side only of each filament contacts the liquid film.
Helical self-crimp also can be imparted to a polyamide fiber by melt spinning the fiber as a composite of two distinct compositions differing in shrinkage characteristics. Such fiers, which are known as bicomponent or conjugate fibers, require more complicated spinning equipment (i.e., extruders, piping and spinnerets) and are more costly and less efficient to produce than ordinary monocomponent fibers.
Several references have disclosed that self-crimping could be provided by melt-spinning and cross-flow cooling of fibers which have cross-sections of special geometry. For example, Hayden, U.S. Pat. No. 3,135,646, suggests that "bulbous or keyhole" cross-sections result in helically crimped fibers. Other types of cross-sections, in which the mass of the fiber is distributed eccentrically around the longitudinal axis of the fiber, have been disclosed by Nakagawa et al., U.S. Pat. No. 3,920,784 and Ono et al., U.S. Pat. No. 3,623,939. Ono et al. also suggest that special cross-sections which provide the fibers with an eccentric shrinkage property with respect to the centroid of the cross-section can be melt-spun at take-up speeds of at least 3000 meters per minute and can produce fine curl-like crimps in the fiber.
Although some of the above-described prior-art techniques can produce helically crimped filaments, applicants have found that such yarns can suffer from "follow-the-leader" crimp, which are bulky per se, but when used in carpets, do not provide the carpet with adequate bulk.
To avoid such problems associated with the prior-art techniques, applicants have invented an efficient, surprisingly simple and energy-conserving sequence of steps that produces a range of novel helically crimped polyamide fibers which generally are suited for use in bulked fiber applications, such as upholstery, and which are suited particularly for use in carpet yarns.